Alternating current ionization chamber



April 4, 1950 R. E. FEARON 2,502,617

ALTERNATING CURRENT IONIZATION CHAMBER Filed June 22, 1948 I PatentedApr. 4, 1950 ALTERNATING CURRENT IONIZATION CHAMBER Robert E. Fearon,Tulsa, Okla., assignor to Well Surveys, Incorporated, Tulsa, Okla., acorporation of Delaware Application June 22, 1948, Serial No. 34,488

9 Claims.

This invention relates generally to the art of geophysical prospectingand more particularly to radioactivity well logging.

Specifically this invention is directed to a method and apparatus formaking an ionization chamber generate alternating current by controllingthe circulation of the ionizable medium in a convection type ionizationchamber, such as that disclosed in my copending application SerialNumber 572,666 now Patent No. 2,472,153.

The convection type ionization chamber employs an ionizable medium suchas argon or helium which contains a trace of a substance, such as xenon,which reduces the electrical mobility of the positive ions withoutaffecting the electrical mobility of the negative ions. This ionizablemedium is circulated continuously in the system by a pump. At one pointin the system the medium is completely de-ionized, then the ionizablemedium is subjected to the gamma radiation that it is desired tomeasure. The gamma radiation entering the ionizable medium producedionization thereof. The area in which the ionization occurs has disposedtherein collector plates to which the negative ions diffuse immediatelywhile the more cumbersome positive ions, by the circulation of theionizable medium, are swept out of this area and into a third area wherede-ionization is afiected. A measurement of the current produced bycollecting either the negative or positive ions is a measurement of thegamma radiation which produced the ionization.

It is obvious, that since the operation of the detector described abovedepends on the propulsion of the ionizable medium, the ionizationcurrent would fall to a much smaller value if the ionizable medium wasat rest. Consequently, if the circulating pump is so arranged that itdelivers a current of the ionizable medium intermittently, the output ofthe electrical system would vary in a corresponding manner. For example,assuming that the operation of the pump occurs in cycles of one secondof pumping and one second of not pumping, it is apparent that the outputof the electrical system will contain a component of alternating currentat one-half cycle per second, inasmuch as two seconds are needed for thecycle which has been ascribed to the pump.

' There are many ways of causing the current of the ionizable medium toflow intermittently. As examples: an ordinary reciprocating pistontypepump, or centrifugal pump of the type employed in sirens, using as theoutput any one of the peripheral ports. Another way of producingintermittent flow of the ionizable medium through the area in the systemin which the gamma rays are detected is by opening and closing a valvein a pipe which acts as a shunt across the circulating pump. If such avalve is closed, the current of the ionizable medium will pass throughthe usual delivery channel of the pump, and if the valve in the shuntline is open, the ionizable medium will pass through the shunt linethereby reducing the delivery, as would be observed in the normaldelivery channel of the pump, to a very small value. Accordinglycyclical openings and closings of the value in the shunt line willproduce cyclical variation in the electrical output which appears fromthe ionization chamber. It is obvious that similar results could beobtained by placing a valve directly in the delivery line of the pumpand opening and closing it cyclically.

The choice of frequency to be employed in interrupting the fiow of theionizable medium would depend upon the spacing of the plates in the zoneof the instrument where the electrons are collected by diffusion. Thefrequency will also depend on the length of this group of platesparallel to the direction of the flow of the ionizable medium. Thechoice of frequency will also be influenced by the diffusibility of boththe positive and negative ions. Generally speaking, the upper limit ofthe required frequency will be fixed by the difiusibility or electricalmobility of the electrons in the working medium, while the lower limitof the operating frequency will be fixed by the difiusibility orelectrical mobility of the positive ions in the ionizable medium. Theamplitude of movement of the medium corresponding with an interval ofpumping should be enough to sweep out the medium from between theworking plates which collects the electrons.

The working plates which collect the electrons will be replaced by asystem of small tubes. An equivalent will also be a mass of steel Woolor shavings, or, in fact, any shaping or conformation of electricallyconducting matter having fine openings between metal surfaces, anddisposing an extensive area of metal surface in a small amount ofvolume.

The way in which the desirable operating frequencies can be fixed, caneasily be understood if the fundamental principle, that everyphoto-electron which occurs in the zone of the electron collectingplates must contribute to the output current, is remembered. Assume forthe of the positive ions have thus been lost most of.

these ions will be swept out of the plates. If the flow had recommencedtoo quickly to permit the electrons to be lost by diffusion theelectrons along with the positive ions would have to be swept out, whichwould result in no net chargebeing carried to the collecting plates andhence no electrical output would result. It is accordingly necessary topermit the ionizable medium to remain at rest sufiiciently long to losea. very large fraction of the electrons which occurred during the rest,period. but not sufiiciently long to. bring about: an appreciable lossof positive ions. The interval of the flow must be approximately. equaLtthe interval of the rest for the greatest; electrical output at thefundamental frequency of. the cyclic process.

There are moreover. other limitations on the frequency choice whichdepend on the intensity of. the. radiation being detected. Intenseradiation willnot be satisfactorilyregistered by low frequency.circulation for the reason that the electronswill not. be lost by.diffusion if too many positiveionsremain during an interval, because of.space) charge efiect of the residual positive charge- Therefore theprincipalobject of this invention.resides. in the provision of a methodand apparatusfon detecting ionizing radiation, such asgamma rays,byproducing an alternating current whose amplitude is a measure of theionizing'. radiation.

Another object of .this invention resides in the provision of anionization chamber. adapted, on exposure of; radiation, to deliveranalternating currentoutput.

Still. another object= of thisinvention, resides in-theprovisioniof aconvection current type ionization. chambe which, when exposed toionizing radiation, will. deliver alternating current.

This invention-alsocontemplates a methodof producing alternating currentin an ionization chamber by, causing the ionizable; medium therein to.circulate. in pulses cyclically.

Another objectof this invention resides in the provision ofnovel means,for detecting extremely weak penetrating radiations, such as gamma rays,by generating an alternating current whose amplitude isclirectlyproportional to the penetrating radiation.

Other objects and advantages of the present invention willbecome.apparent fromthe following detailed; description when considered with eur wine which.

Figure Us a. diagrammaticillustration of one formofthepresentinventionillustrating the use of a. pump for circulatingtheionizable medium by pulses;

Figure 2 is a diagrammaticillustrationof the second form of the. presentinvention showing the use. of a valve in the. supply line from the pumpfor controlling the pulses in the ionizable medium;

Figure 3 is a diagrammatic illustration of a third form of the instantinvention illustrating the use of a valve in the shunt line of a pump;and

Figure 4 diagrammatically illustrates a still further modification ofthe present invention in that the multiple detectors are employed inproducing the alternating current.

Referring to the drawings in detail particularlyv Figure 1, there isillustrated an ionizable medium circulating system I0, having a pump ii,that is adapted on operation to circulate the ionizable medium in thedirection indicated by the arrows. The ionizable medium may consist of agas, such as argon, plus a trace of propylamine; The trace of Xenon isadded to the gas to reduce the electrical mobility of the positive ions.The gas being circulated in the form of pulses by thepump. II, passesthrough a set of de-ionizing plates I2, of which every other plate ismade positive with respect to ground by means of a battery I3. Thoseplates which are not positive with respect to. ground, are connected tothe ground and'to -the negative terminal of the battery I3. Theelectricalficld:between the plates I 2 is made sufliciently strong tosweep all ions from the circulating gas. The gas flowing through theplates I2; is thus completely deionized, and on reaching the zoneoccupied by a second set of plates I4, becomes subjected to thegammaradiation that. it is desired to measure. The gamma radiation.causes the gasto become ionized. Since the electrons have considerablymore difiusibility than the positive ionsthey will be. immediatelyabsorbed by the plates I4. The reduced diffusibility. or electricalmobility. of 1 the positive-ions, due'to-the propylamine, will causethem to be carriedvout of. the zone occupied by the plates. I4",intothezone occupied by a third set: ofplates I51 Every other one. ofthe plates I5; are connected to-thepositive side of the battery I6, andthe remaining-platesare connected to the negative:side-of the-batteryI6, andto ground. The plates I5, in the manner described inconnectionwith theplates I2; also function as means=for de ionizing-thegas asitpassesover them.

The electronsabsorbed by plates III-produce a flow of current intheconductor l'l; through the input of amplifier I8-to-ground-at I9. Due tothe fact: that: no surge-- chamber is provided the gas-willflowin pulsesand as a result the current produced will be a pulsating current havinga frequency that is equal tothe number of strokes of the pump II persecond-.- The alternating component ofthe: signal-thusproduced in theinput-of amplifier-I 8 is amplified at the frequency ofalternation ofthe pump and indicated or recorded by the meter-or recorder-2 0' as ameasure of. the gammaradiation entering-"the region of the plates- I4:

In-Fig u-re-Z there-is illustrated a modification of the-presentinvention. This-form of the'invention differs from that shown in Figure1 inthat a'valve 2! is disposedin the ionizable medium circulatorysystem' I Il-onthe exhaust sideof the pump lI. ValveZI is-caused-toopenand close ata predeterminedrate by the synchronous m0- to 22 whichdrives it through the-gearbox 21. Gear-box 23 is equipped with aplurality; of gears whereby the speed at which valve 2I is opened andclosed can be varied at will to control the frequency of pulsationof thegas flow-and thereby control the frequencyof the signal amplified andrecorded-by the amplifier I8 and the recorder 20.

Still another embodiment-of'this invention is illustrated in Figure-3.In'thls form of the invention the circulatorysystem I is provided with aby-pass 24 which permits the gas being circulated by the pump II toby-pass the plates l2, I4 and i5. A valve 25 is disposed in the by-pass24. Valve 25 may be driven by a synchronous motor 26 through a gear box21 in the same manner as described in connection with Figure 2 toproduce pulsations in the flow of gas over the plates l2, l4 and [5. Inthis form of the invention, as well as'those illustrated by Figures 1and 2, the useful signal is produced by diffusion of electrons to theplates l4.

Another embodiment of the present invention is illustrated in Figure 4.In this form of the invention the circulatory system I0 is arrangedto'provide two like paths 26 and 21 for the flow of gas. Each path isprovided with a similar set of plates which function in the same manneras those described above inconnection with Figures 1, 2 and 3. Theplates in path 26 have the same reference characters as used in thepreceding figures and the plates in path 21 have the same characterswith primes added. The battery I3 is used for both sets of plates [2 andi2 and the battery I6 is used for both sets of plates I5 and The plates14 and I4 are connected to opposite sides of the input of a push-pullamplifier 28 which in turn is connected to the recorder or indicator 29.

In the operation of this form of the invention r the pump ll continuallyforces the gas toward a valve 30 which is disposed in a Y formed by thecirculating paths 26 and 21 and the path leading from the pump. Valve 30may be of the rotary type and be driven by a synchronous motor 3|through a speed control gear box 32. The ports in the valve are soarranged that the valve intermittently passes the current of gassupplied by pump II into path 26 and path 21. The currents produced bydiffusion of electrons on the separate groups of plates l4 and M arebrought by separate leads to the opposite sides of the push-pullamplifier 28 which impresses an alternating current signal at thefrequency of operation of the valve 30 on the indicator or recorder 29which indicates or records the intensity of the alternating currentsignal as an indication of the gamma rays falling on the two sets ofplates 14 and I4.

Although the present invention finds broad application in industry it isto be understood that it is particularly applicable to well surveyingsystems. The manner of incorporating the present invention in a wellsurveying system, except for small changes, is the same as thatdisclosed in my copending application Serial Number 572,666.

I claim:

1. A detector of ionizing radiation comprising in combination meansdefining a closed circulatory system, at least a portion of which isadapted to admit ionizing radiation, an ionizable medium in said system,means for circulating said medium in the system to expose it to theadmitted radioactive ionizing emanations, said means being adapted toimpart surges to said ionizable medium that are separated by uniformtime intervals, means for de-ionizing said medium before it enters theportion adapted to admit ionizing radiation, means for separating theions produced by said radiation when the medium enters that portion,means for collecting one group of the separated ions, and means formeasuring the alternating component of the electrical current producedby the collection of said ions as a measure of radiation.

2. An apparatus for detecting penetrating radiation that comprises incombination a sealed system, an ionizable medium in, said system, meansfor cyclically circulating said medium in said system, said means beingadapted to impart surges to said medium which are separated by uniformtime intervals, means in the path of said medium for de-ionizing saidmedium, said system having a zone in which the penetrating radiationenters the ionizable medium to produce ionization of the de-ionizedmedium, means adjacent said zone for collecting the negative ions, meansat another point in the path of circulation of said surging medium forsubsequently collecting the positive ions, and means for amplifying andrecording the alternating component of the current produced by thecollection of the negative ions.

3. An apparatus for detectingpenetratingv radiation that comprises incombination a sealed system, an ionizable medium in said system, meansfor circulating said medium, said circulating means being adapted toimpart surges to said medium separated by uniform time intervals, meansin the path of said medium for deionizing said medium, means forseparating positive and negative ions formed by subjecting theole-ionized medium to penetrating radiation, and means for recording thealternating component of the current produced by collecting theseparated negative ions as an indication of the intensity of thepenetrating radiation.

4. An apparatus for detecting penetrating radiation that comprises incombination a sealed system, an ionizable medium in said system, meansfor circulating said medium, said circulating means being adapted toimpart surges to said medium separated by uniform time intervals, meansin the path of said medium for de-ionizing said medium, means forseparating positive and negative ions formed by subjecting the deionizedmedium to penetrating radiation, and means for recording the alternatingcomponent of the current produced by collecting one group of theseparated ions as an indication of the intensity of the penetratingradiation.

5. A detector of radioactive emanations comprising in combination aclosed circulatory system; an ionizable medium in said system; means forcirculating said medium in said system; said circulating means beingadapted to impart surges to said medium that are spaced from each otherby uniform time intervals; said circulatory system having in successionin the path of the circulating ionizable medium, a de-ionizing zone, anionizing zone, and a second ole-ionizing zone; deionizing means in eachof said de-ionizing zones; means in said ionizing zone for separatingthe positive and negative ions produced by the action of radioactiveemanations of said ionizable medium; means for collecting one group ofthe separated ions; and means for recording the alternating component ofthe electrical current produced by the collection of one group of ionsas a measure of the radioactive emanations.

6. An apparatus for detecting penetrating radiation that comprises incombination a sealed system, an ionizable medium in said system, a pumpin said system for circulating said medium in surges that are spacedfrom each other by uniform time intervals, means in the path of saidmedium for de-ionizing said medium, means for separating positive andnegative ions formed by subjecting the de-ionized medium to the pene- 7trating radiation, means for collecting the negative ions, and means forrecording the alternating component of the current produced bycollecting the separated negative ions as an indication of the intensityof the penetrating radiation.

7. An apparatus for detecting penetrating radiation that comprises incombination a sealed system, an ionizable medium in said system, meansin said system for circulating said medium in said system, means in thepath of said medium for separating positive and negative ions formed bysubjecting the medium to the penetrating radiation, means forming a partof the sealed system defining a medium path that shunts the separatingmeans, means for intermittently diverting the circulating medium to theshunt path, means for collecting the separated negative ions, and, meansfor recording the alternating component of the current produced bycollecting the separated negative ions as an indication of the intensityof the penetrating radiation.

8. An apparatus for detecting penetrating radiation that comprises incombination a sealed system, an ionizable medium in said system, saidsealed system having means defining two circuits for said ionizablemedium, means for alternately circulating said ionizable medium throughthe circuits, separate means in each circuit for separating positive andnegative ions formed by subjecting the ionizable medium in each circuitto the penetrating radiation, means for collecting the negative ionsproduced in each circuit, and means for conducting the current producedby collecting the separated negative ions in each circuit to oppositesides of a push-pull amplifier, and means for recording the outputalternating current from the amplifier as an indication of the intensityof the penetrating radiation.

9. An apparatus for detecting penetrating radiation that comprises incombination a sealed system, an ionizable medium in said system, meansfor circulating said medium, said circulating means being adapted toimpart surges to said medium separated by uniform time intervals, meansin the path of said medium for deionizing said medium, means composed ofeleotrically conducting matter with small openings in it through whichthe ionizable medium circulates,such means to receive the radiationwhich it is desired to measure, and to which means the free electronsproduced by the ionization will difiuse, but to which, because of lackof suflicient time, the positive ions will not diffuse the deionizedmedium to penetrating radiation, and means for recording the alternatingcomponent of the current produced by collecting one group of theseparated ions as an indication of the intensity of the penetratingradiation.

ROBERT E. FEARON.

No references cited.

